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Brief Review

The Acute Effects From the Use of Weighted Implements on Skill Enhancement in Sport: A Systematic Review

Jermyn, Sam; O'Neill, Cian; Coughlan, Edward K.

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Journal of Strength and Conditioning Research: October 2021 - Volume 35 - Issue 10 - p 2922-2935
doi: 10.1519/JSC.0000000000004109
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Abstract

Introduction

The development and improvement of motor skills is an integral factor in the attainment of sporting success (16,38). Consequently, athletes, coaches, and researchers are on a constant quest to explore and identify the most effective means of enhancing sport-specific motor skills. In particular, because of recent research and programming trends placing greater emphasis on specificity and efficient training modalities, a substantial volume of empirical research has been conducted on the acute and chronic effects of weighted implements over the past 3 decades (4). Weighted implement training involves exercising with modified sporting implements while duplicating the force-velocity output and full range of motion of the respective motor skill (6,7). This includes swinging a baseball bat that is heavier or lighter than regulation weight or throwing an overweighted or underweighted baseball to prime the accelerative properties of a desired action in preparation to compete. As weighted implement training maintains high specificity between the weighted movement and the original movement (9), it has been categorized as a form of specific resistance training, in which the principle of speed overload (underweighted implements) or force overload (overweighted implements) can be applied (10,46).

Research assessing the efficacy of weighted implement training as a means of performance enhancement dates back to the early 1960s (50). The aim of this seminal research, as well as the subsequent research to date, has been to investigate the potential of weighted implement training to enhance key variables underlying the successful outcome of sport-specific motor skills, with a prevalence for assessing their effects on pitching and batting velocity (4,9,12,43,46,47). Some studies have also assessed the effects of a weighted football on kick distance (1) and a weighted cricket ball on speed of bowling (51). However, these studies collectively report inconsistencies pertaining to the effectiveness of chronic weighted implement training programs at enhancing motor skills. Therefore, although various studies since the turn of the 21st century have investigated the training effects of weighted implements (4), the aforementioned inconsistencies pertaining to the effectiveness of weighted implement training programs have resulted in a considerable portion of weighted implement training research investigating its acute effects (26). These studies that have investigated the acute effects of weighted implement training have, therefore, investigated the efficacy of using weighted implements as a warm-up strategy. A prominent purpose of these studies has been to investigate the acute effects of underweighted and overweighted balls and bats on baseball pitching and batting velocity. Studies have investigated the acute effects of multiple readily available implements of varying mass and moments of inertia on baseball pitching and batting, with pitching accuracy and pitching and batting velocity, being the most commonly assessed performance variables. In many throwing and pitching sports, enhancing throwing velocity and accuracy is considered integral to competitive success. For example, the development of speed and accuracy among baseball players may result in less time for opponents to react to an on-target pitch, therefore enhancing the pitcher's potential to strike out the batter (4). Similarly, increasing cricket or softball outfield players' ability to throw at higher speeds with greater accuracy may increase the likelihood of completed passes to teammates and enhance the chances of eliminating an opponent. With regard to batting performance, enhancement of bat velocity is considered integral to performance as increased velocity values result in a faster swing time. This provides batters with greater time to obtain information from ball flight due to the affordance of delaying swing onset times, subsequently producing greater batted-ball distances and velocities (46).

Several research studies have compared motor skill performance with regulation and weighted implements (14,22,49,51). Although these studies have not included follow-up measures of performance on return to the regulation implement, their findings provide an insight into the potential implications of using the respective implement immediately before competitive performance. Consequently, athletic performance stakeholders, such as strength and conditioning (S&C) coaches and skill acquisition specialists, are provided with substantial evidence pertaining to the acute effects of weighted implements. Such studies have also made varying suggestions as to the optimal programming of weighted implement training to induce immediate enhancements of motor skill performance. Therefore, S&C coaches and skill acquisition specialists are provided with an abundance of findings, and corresponding suggestions pertaining to their programming, which need to be synthesized.

Because of the importance of sport-specific motor skill proficiency in sporting success (25), identifying and using effective preparatory tools and routines is a priority for practitioners and athletes, leading to this comprehensive analysis of the existing literature. To date, the weighted implement training literature has been of greater interest to baseball; however, there is also a need to identify sports that require investigation into the acute effects of weighted implement training on respective motor skill performance. Therefore, the purpose of this systematic review is 2-fold: (a) to assess the acute effects of weighted implement training on sports performance, providing a comprehensive overview of the research to-date, and (b) to provide recommendations to guide future research in the weighted implement training field.

Methods

Experimental Approach to the Problem

A systematic review was conducted in accordance with the PRISMA (Preferred Recording Items for Systematic Meta-Analyses) guidelines (28). Terms including “weighted implement,” “modified implement,” “weighted football,” “heavy bat,” “weighted club,” “weighted racquet,” and “weighted baseballs” were searched across 6 databases including Google Scholar, EBSCOhost, MEDLINE, SPORTDiscus, Science Direct, and Academic Search Complete. The search focused on the literature from inception to November 2019 to allow for the potential inclusion of all relevant studies.

Procedures

Following the online systematic screen outlined in Figure 1, duplicate studies were removed with a subsequent screen of the remaining studies' abstracts. The full-text articles were retrieved, and a comprehensive review was conducted to ensure that all inclusion criteria were adhered to. Additional analyses of reference lists of included articles were conducted, with direct contact established with authors of identified studies that were not accessible online. From each identified study, the following experimental features were extracted and analyzed: the subject population, standard implement weight, weight deviation of the modified implement from standard weight, and subsequent effect on all performance variables with standard implements. The inclusion criteria for this review were set a priori, requiring studies to (a) include an investigation into the acute effects of a sport-specific weighted implement on immediate sport-specific motor skill performance with the regulation implement, (b) be published in a peer-reviewed journal or relevant conference proceedings, (c) include a weighted implement warm-up protocol and immediate post-test, (d) have an adult or youth subject cohort, and (e) be written in English. Studies were excluded if only a comparative analysis of motor skill performance between the weighted implement and regulation implement was performed, and also if the conditioning activity used conventional resistance training equipment including barbells, dumbbells, manual and air resistance machines, cable machines, medicine balls, and kettlebells.

Figure 1.
Figure 1.:
Flowchart summarizing the screening and selection process (28).

Results

A total of 1,653 articles were retrieved on initial searches, of which 116 were selected for further review. Following the removal of duplicates, 75 studies remained, of which 48 were excluded following the screening of abstracts. This resulted in a total of 27 studies. The full texts of these articles were reviewed. Six of these articles were subsequently excluded for reasons that included a case study design (n = 2), an uneven sample of experimental trials per weighted implement condition due to invalid data (n = 1), not stating the weight of the implements used (n = 1), the inclusion of a non–sport-specific motor skill (n = 1), and absence of a post–warm-up test (n = 1). Four additional studies were identified on screening of the included studies' reference lists. Subsequently, 25 studies, meeting the inclusion criteria of this review, were included (Figure 1). In the Discussion section of this review, all selected studies are categorized based on the type of respective implement used: underweighted and overweighted balls (n = 5), weighted indoor weight throw implements (n = 2), and underweighted and overweighted bats (n = 18). Studies that used weighted bats are categorized into further subsections.

Four weighted ball studies involved the use of under and overweighted baseballs, with 1 study investigating the effects of weighted cricket balls on bowling performance. Seventeen dependent variables were reported, of which throwing/pitching velocity was most frequently investigated. The 2 studies investigating the acute effects of weighted indoor weight throw implements involved the use of overweighted indoor weight throw implements. Three dependent variables were reported in each study. Of the 18 studies that used a weighted bat, 17 included the use of underweighted and overweighted baseball bats and overload attachments, whereas 1 study investigated the use of underweighted and overweighted softball bats and overload attachments among a softball cohort. Forty-six dependent variables were examined across these studies, with bat velocity being the most frequently investigated marker of batting performance. The results of this review process highlight a clear paucity of research investigating the acute effects of other weighted implements, such as weighted footballs, rugby balls, basketballs, golf clubs, hockey sticks, and racquet implements. The authors acknowledge that several included studies are relevant to multiple sections within the discussion (for example, Weighted Ball Studies and Underlying Mechanisms).

Discussion

Table 1 illustrates the included studies that investigated the acute effects of underweighted and overweighted balls on pitching and bowling performance with respective regulation implements (n = 5). The greatest decrease in ball weight from the respective study's regulation ball was 20% (40), with the greatest increase being 200% (42). All 5 studies investigated the effects of such implements on ball velocity, with additional variables measured including pitch/bowling accuracy, muscular activation, and perception of throwing speed and ball weight.

Table 1 - Acute effects of underweighted and overweighted balls on throwing performance.*
Reference Sport Subjects Implements (% increase/decrease from standard) #WU trials; rest period between WU and post-WU; post-WU intertrial rest Results and conclusions
Van Huss, Albrecht, Nelson, and Hagerman (50) Baseball 50 university freshman baseball players R 142 g (5 oz) OB 312 g (11 oz, 120% ↑) 25 (15 submaximal and 10 maximal effort throws); minimal; N/A OB significantly ↑ V
OB did not significantly ↑ A but gradually ↑ A
Straub (42) Baseball 60 male high school students R 142 g (5 oz)
OB 284 g (10 oz, 100% ↑)
OB 425 g (15 oz, 200% ↑)
20; N/A; N/A No significantly ↑ V
No significantly change to A
Morimoto, Ito, Kawamura, and Muraki (30) Baseball 8 male university baseball players R 142 g (5 oz)
UB 128 g (4.5 oz, 10% ↓)
OB 156 g (5.5 oz, 10% ↑)
6 or 18; minimal; N/A Significantly ↑ V following 6 and 18 throws with UB and 18 combined throws
No significant changes in A
“Lightness” and “ease of pitching” after OB warm-up
“Heaviness” and “difficult to pitch” after UB warm-up
Shin and Choi (40) Baseball 12 high school and university pitchers R 142 g (5 oz)
UB 113 g (4 oz. 20% ↓)
OB 227 g (8 oz, 60% ↑)
10; 10 min; N/A No significant differences in ball V following each warm-up protocol but each ball caused varying muscular activation
Feros, Young, and O'Brien (13) Cricket 17 male community-level pace bowlers R 156 g (5.5 oz)
OB 300 g (10.6 oz, 92.31% ↑) and OB 250 g (8.8 oz, 60.26% ↑)
18; 3 min; post-test = 4 overs (4 × 6 throws)–3 min between overs, 30 s between trials No significant difference in V
OB condition significantly ↓ A vs. RB
*V = velocity; A = accuracy; R = regulation ball; OB = overweight ball; UB = underweight ball; RB = regulation ball; N/A = not available; ↑ = weight increase; ↓ = weight decrease.

Although all included studies investigated the effects of weighted balls on throwing velocity following a weighted ball warm-up, 4 studies also assessed the impact of such protocols on throwing accuracy with a regulation ball. Two baseball studies reported a significant increase in pitching velocity following a weighted baseball throwing protocol (30,50). Only 1 study reported a subsequent improvement in accuracy (50). However, as accuracy improvements were not statistically significantly different between groups, it is advised that the observed improvements in accuracy following pitches with the weighted baseball should be interpreted with caution.

Van Huss et al. (50) reported a significant improvement (p = 0.01) in pitching velocity from pre–warm-up to post-test following a weighted ball warm-up among 50 collegiate freshman baseball players (each serving as their own control). Velocity of throws in the post–overload warm-up consistently increased until the sixth post-trial before plateauing. Pitching velocity was measured with a chronoscope (product manufacturer details not specified). Although accuracy diminished in the first 7 trials post–weighted ball warm-up in comparison to the standard ball, accuracy of the final 3 post-test trials was greater than that of the standard warm-up. Accuracy was measured based on the ball's contact with a target in which various rectangular segments were arranged. A 1–5 scoring system based on the point of contact determined pitching accuracy.

Later in the decade, Straub (42) investigated the acute effects of a standard baseball and 2 overweighted baseballs on the velocity and accuracy of the overarm throw. Based on an initial velocity assessment, 60 high school subjects were equally divided into “high-velocity” (n = 30) and “low-velocity” (n = 30) groups, with each group comprising of 3 subgroups of 10 subjects. Results demonstrated that the overload warm-ups did not result in significant improvements (p > 0.05) in throwing velocity at either velocity levels, with the regulation warm-up resulting in similar velocities. Pitching velocity was measured with the use of a chronoscope (product manufacturer details not specified). The overweighted implement conditions, and regulation implement condition, did not produce any significant changes (p > 0.05) in pitching accuracy for any group. Pitching accuracy was measured based on the ball's contact point with a target, in which concentric circles on the target indicated various accuracy zones. A scoring system was used to depict accuracy.

The aforementioned studies (42,50) exclusively used overweighted baseballs of substantially greater mass than a regulation baseball (≥100% weight increase). In contrast, Morimoto et al. (30) assessed the immediate effects of overweighted and underweighted balls that were slightly different from the standard weight on ball speed and accuracy. Eight male university players were required to pitch 128 g (4.5 oz), 142 g (5 oz), or 156 g (5.5 oz) baseballs. Each ball was pitched 6 or 18 times. A combination condition, which required 6 or 18 pitches with all 3 balls (order of 156 g/5.5 oz, 142 g/5 oz, and 128 g/4.5 oz), was also performed. Post-test velocities of the regulation baseball were significantly faster (p < 0.01) following 6 and 18 throws with the light ball and 18 throws under the combined condition. However, it was reported that ball speeds following the 18 combined conditions were similar to velocity values measured following 6 throws with the light ball, suggesting that there may have been minimal effect from the heavy or standard ball before pitching the light ball. Morimoto et al. (30) suggested that the number of throws is of great importance as the 6-trial combined condition, which concluded with 2 throws with the underweighted ball, resulted in no significant improvement (p > 0.05) in pitch velocity. Pitching velocity was measured with a speed gun (Mizuno, PSK-DSP; full product manufacturer details not reported by authors). Similar to the findings of Straub (42), accuracy scores revealed no significant difference (p > 0.05) among all conditions. Pitching accuracy was determined based on the ball's contact point with a target in which an “X” was marked in the center. Video analysis of the coordinates of this contact point and the target center were used to determine the displacement of each pitch (i.e., accuracy). Subjects reported sensations of “heaviness” and “difficult to pitch” during post–warm-up throws following the underweighted ball and feelings of “lightness” and “ease of pitching” during post–warm-up pitches following the overweighted ball. Thus, although slight, the 28.35 g (1 oz) difference in weight between the underweighted and overweighted baseballs may induce altered kinesthetic perceptions of pitching performance on return to the regulation baseball weight.

More recently, Shin and Choi (40) found no significant differences (p > 0.05) in post–warm-up pitching velocity of a regulation baseball following warm-up throws with a standard, underweighted, or overweighted baseball. Pitching velocity was measured with a radar gun (Sport radar, 24.7 GHZ, SP78585, Applied Concepts, Inc., Northbrook, IL). Muscle activation of the internal and external shoulder rotators of the pitching arm was also measured using a wireless electromyography (EMG) device (SENIAM Guide Line; full product manufacturer details not reported by authors). Electromyographic activity of the biceps brachii and triceps brachii was also assessed. Although no significant differences (p > 0.05) in pitching velocity were found, activation of various muscles surrounding the shoulder slightly varied dependent on warm-up condition. As shoulder internal rotational velocity and elbow extension velocity account for 67% of ball velocity at release (48), it was surprising that increased activation in the muscles responsible for these variables (for example, anterior deltoid, latissimus dorsi, and triceps brachii) following weighted ball warm-ups was not matched by significantly greater pitching velocity in post-test trials. Therefore, the authors attributed such activation increases to compensations from decreased pitching velocities that were exhibited when warming up with the weighted ball. It was concluded that the weight of the ball used in warm-ups should be selected based on individual status, with the variation in muscular activity dependent on the weight of the ball having potential implications for the design of long-term weighted ball training programs.

In the only cricket study included in this review, Feros et al. (13) found no significant difference (p > 0.05) in velocity and perception of effort following standard ball and overweighted ball warm-ups among 17 community-level bowlers. Bowling velocity was measured with a radar gun (Stalker Pro; Applied Concepts, TX). Although each condition did not affect bowling velocity, it was found that the weighted warm-up condition resulted in significantly lower mean accuracy (p = 0.049) as subjects experienced a 10.9% decrease in accuracy compared with that of the control condition. The effect size (ES) of this difference was d = 0.581, which indicates a medium ES (34). Bowling accuracy was assessed by use of a vertical target sheet, which was placed in line with the stumps at the batsman's end of the pitch. With the use of Dartfish Connect Version 6 (Dartfish, Australia), the distance between ball strike and the crosshair of the target was calculated to determine accuracy.

This review process illuminated a dearth of research into the acute effects of underweighted and overweighted balls on throwing and kicking performance in sports such as soccer, rugby, Gaelic football, Australian rules football, American football, basketball, and Olympic handball. Of the existing weighted implement training literature regarding weighted balls, the current review process highlighted multiple studies assessing the chronic effects of weighted ball throwing programs, as only a small proportion of the respective literature has investigated the acute effects of this training modality on overarm throwing and pitching. Of these studies, an overriding prevalence of investigations into the acute effects of weighted baseball implements is apparent. Within the existing literature, there are conflicting reports of the acute effects of weighted baseballs on pitching velocity. Although it has been demonstrated that warm-ups with 127.58 g (4.5 oz) and 311.85 g (11 oz) baseballs result in significant improvements in pitching velocity, it has also been shown that warm-ups with 113.40 g (4 oz), 155.92 g (5.5 oz), 226.80 g (8 oz), 283.50 g (10 oz), and 425.24 g (15 oz) baseballs do not lead to significant improvements. Because of substantial differences in methodological design, including total throws, ball weight, and subject playing levels, comparison between studies is a complex process. Therefore, this accentuates the need for future research to specify optimal programming to induce immediate improvements in performance. However, as indicated by Morimoto et al. (30), the amount of warm-up trials with a weighted implement seems to have an effect on the magnitude of pitching velocity improvement, a variable that should be further investigated. With specific regard to throwing accuracy, the collective findings of included studies investigating the effects of weighted balls on this variable imply that 6 differently weighted balls have a negative effect on, or induce no significant changes to, post–warm-up pitching accuracy. With the use of a wireless EMG device, it was also shown that weighted balls induce varying muscular activation rates on return to a regulation baseball (40), an effect that should be closely considered in the design of chronic weighted baseball throwing programs. Future research should also assess the impact of various rest intervals post–weighted implement training warm-up.

Two studies investigated the acute effects of weighted indoor weight throw implements on indoor weight throw performance. The indoor weight throw is an indoor track and field event, similar to that of the hammer throw event that is performed in the outdoor setting (19). These studies investigated the acute effects of these implements, with both incorporating a control condition (standard implement warm-up), on mean and peak throw distance with the standard implement, and post–warm-up fatigue. The procedure of both studies required subjects to perform 5 one-heel turn throws with the randomly assigned implement, followed by 3 maximal effort throws with the standard implement. A 3-minute rest was completed between each post–warm-up attempt.

Judge et al. (18) found that 5 throws with an overweighted indoor weight throw implement weighing 1.37 kg (p = 0.004) or 2.27 kg (p = 0.027) heavier than standard male (11.4 kg) and female (9.1 kg) implements significantly improved peak throw distance compared with that of a standard implement condition. Peak throw distance was the greatest distance thrown from 3 maximal effort throws with the standard implement, whereby a 3-minute time interval elapsed between the intervention trials and the postintervention trials, as well as between each postintervention trial. The +1.37 kg condition meant that the male implement weight was 12.77 kg (12.02% weight increase), whereas the female weight was 10.47 kg (15.05% weight increase). The +2.27 kg condition meant that the male implement weight was 13.67 kg (19.91% weight increase), whereas the female weight was 11.37 kg (24.95% weight increase). The subject cohort comprised of high school indoor weight throw athletes (n = 10). There were no significant differences in peak distance between the 2 overweighted conditions (p > 0.05). No significant differences (p > 0.05) in mean throwing distance were evident following throws with both overweighted conditions, although mean distance was greatest following the +1.37 kg condition. Although Judge et al. (18) found that the +1.37 kg (p = 0.025) and +2.27 kg (p = 0.007) resulted in significantly greater sensations of fatigue post–warm-up, resultant throw distance indicated that this did not degrade performance. Indeed, the lighter of the 2 overweighted implements induced a 0.9-m increase in peak throw distance compared with the standard implement. As peak throw distance following throws with the standard implement was 14.15 m, the 0.9-m increase (6.36%) following the lighter of the 2 overweighted implements may have considerable performance implications in competition.

Bellar et al. (2) also found overweighted indoor weight throw implements weighing 1.37 and 2.27 kg heavier than standard male (15.87 kg) and female (9.07 kg) implements significantly improved peak throw distance. The +1.37 kg condition meant that the male implement weight was 17.24 kg (8.63% weight increase), whereas the female weight was 10.44 kg (15.1% weight increase). The +2.27 kg condition meant that the male implement weight was 18.14 kg (14.3% weight increase), whereas the female weight was 11.34 kg (25.03% weight increase). This study comprised of collegiate and elite indoor weight throw athletes (n = 17). The authors reported that mean throw distance following the +1.37 kg condition (p ≤ 0.01, ES = 1.49) and the +2.27 kg condition (p ≤ 0.01, ES = 1.09), compared with the standard warm-up, was significantly greater during the first post–warm-up throw. The authors reported an ES calculated in accordance with partial eta squared (ηp2), which indicates the proportion of variance of the dependent variable that is explained by the independent variable. A partial eta squared output (percentage of variance explained) of ≥0.01–<0.06, ≥0.06–<0.138, or ≥0.138 indicates a small, medium, or large ES, respectively (34). Mean throw distance of the second post–warm-up throw was also significantly greater (p = 0.007, ES = 0.762) following the +1.37 kg implement warm-up condition compared with that of the standard condition. Mean distance of all post–warm-up throws was significantly lower (p < 0.02, ES > 0.8) following the standard implement than the other conditions, with peak post–warm-up throw distance being significantly greater than the standard condition following the +1.37 kg (p < 0.002, ES = 1.01) and +2.27 kg (p < 0.044, ES = 0.619). No significant differences were evident between the 2 overload warm-ups for peak (p = 0.768, ES = 0.08) and mean (p > 0.05) throw distance.

The findings of these studies imply that an overweighted implement warm-up enhances indoor weight throw performance among high school (18), and collegiate and elite (2), indoor weight throwing athletes. Collectively, these studies imply that the +1.37 kg implement (i.e., a 12.02 and 15.05% weight increase for male and female high school athletes, respectively, and a 8.63 and 15.1% weight increase for male and female collegiate and elite athletes, respectively) may induce greater performance compared with the other implements, although the increases following both overweighted warm-ups may result in significant changes in the competitive setting.

Table 2 presents the included studies that investigated the acute effects of underweighted and overweighted bats on performance with a standard 30 oz bat (n = 7). These studies included warm-up implements ranging from 300 g (10.6 oz, 64.67% weight decrease) to 2,722 g (96 oz, 220% weight increase). All 7 studies investigated the effects of such implements on bat velocity, with additional variables measured including bat kinematic variables such as peak bat acceleration and peak velocity (PV) at peak acceleration (PA). Subjects were required to perform dry swings or swings at a stationary target when performing swings post–weighted bat warm-up.

Table 2 - Acute effects of weighted bats on batting performance with an 850 g/30 oz bat.*
Reference Sport Subjects Implements (% weight increase/decrease from standard) #WU trials; rest period between WU and post-WU; post-WU intertrial rest Results and conclusions
DeRenne (5) Baseball 23 male college, ex-college, and ex-professional players L = 652 g (23 oz), 709 g (25 oz), 765 g (27 oz)
H = S+: 794 g (28 oz) donut, “on-deck power swing,” H
3; immediate; minimal (rotational basis within group) 765 g (27 oz) bat and slightly overweighted bat produced greatest BV; 652 g (23 oz) bat and donut attachment had adverse effects on BV
DeRenne and Branco (8) Baseball 20 male college players  12 devices from 652 g (23 oz, 23.33% ↓)–1,446 g (51 oz, 70% ↑) 4; immediate; immediate 709 g (25 oz) bat produced greatest BV with donut, power sleeve, and power swing resulting in greatest ↓BV
DeRenne et al. (11) Baseball 60 male high school varsity players 13 devices from 652 g (23 oz, 23.33% ↓)–1,758 g (62 oz, 106.66% ↑) 4; minimal; 20 s Approximately ±10% of 850 g (30 oz) S produced greatest BV
Bats >964 g (34 oz) and <765 g (27 oz) produced significantly slower BV vs 765–964 g (27–34 oz) bats; 652 g (23 oz), 1,446 g (51 oz) and donut produced slowest BV
Higuchi et al. (15) Baseball 24 collegiate players S 850 g (30 oz)
H 1,531 g (54 oz, 80% ↑)
Swing-specific isometric contraction condition (iso)
3 (included isometric contraction condition); 1 min; 10 s S did not significantly change standard bat BV
1,531 g (54 oz) bat produced a significant ↓ in standard bat BV; iso acutely ↑BV
Szymanski et al. (45) Baseball 22 Division 1 collegiate players 10 devices from 624 g (22 oz, 26.66% ↓)–2,722 g (96 oz, 220% ↑) 3; 20 s; 20 s No significant difference in BV following WU with any of the 10 bats
Wilson et al. (53) Baseball 16 Division II intercollegiate players 5 devices ranging from L 737 g (26 oz, 13.33% ↓)–H 1,417 g (50 oz, 66.66% ↑) 5; post-trials at 1, 2, 4 and mins post-WU No significant weight effects on any of the velocity or acceleration variables, but pooled data revealed PV, PA, and PVPA significantly ↑ between 4 and 8 min
Williams et al. (52) Baseball 15 varsity players S 850 g (30 oz)
L 301 g (10.6 oz, 64.67% ↓)
H 1,576 g (55.6 oz, 85.33% ↑) donut condition
Weighted glove condition
5; 1 min; 20 s No significant differences between bats for effects on MRV, RVBC, time between MRV and RVBC, and bat angle at MRV and RVBC
73.3% of subjects preferred donut condition
*L = light bat; H = heavy bat; S = standard bat; BV = bat velocity; WU = warm-up; MRV = maximal resultant velocity; RVBC = resultant velocity at ball contact; PVPA = peak bat velocity at peak acceleration; PV = peak bat velocity; PA = peak bat acceleration; ↓ = decrease; ↑ = increase.

DeRenne et al. (5,8,11) investigated the acute effects of a variety of overweighted and underweighted baseball bats on immediate bat velocity of a standard bat. The 1982 and 1992 studies measured bat velocity with a photosensing computerized timer (product manufacturer details not specified), whereas the 1986 study measured bat velocity with an accelerometer device (product manufacturer details not specified) that was attached to the barrel of the bat and an accompanying Bat Swing Profile Computer. These studies consistently found the addition of a donut ring to the standard bat produced the lowest bat velocity, with said findings having implications for the addition of a weight at the distal end of the bat. In addition, it was consistently found that bats of minimal deviation from the standard bat weight produced greatest bat velocities, leading to the recommendation that slightly underweighted and overweighted bats (i.e., ±12% of a standard 850 g/30 oz baseball bat) may be optimal to immediately enhance bat velocity.

Higuchi et al. (15) found no significant change (p > 0.05) in bat velocity following a standard bat warm-up condition among 24 collegiate baseball players. Bat velocity was also measured with a computerized photosensing timer device (BatMaxx 5500; Technasport, MN). However, a significant decrease in bat velocity (p < 0.05) was evident following an overweighted bat warm-up (680 g/24 oz Pow'r Wrap). Although this finding of a negative impact of an overweighted bat aligns with the work of DeRenne et al., numerous studies have reported dissimilar results that contrast the recommendation that bats should be ±12% of a standard bat weight to enhance bat velocity.

Szymanski et al. (45) found no significant differences (p > 0.05) in bat velocity of a standard bat following warm-up swings with 10 differently weighted bats. In this study, bat velocity was measured with a Setpro SPRT5A chronograph (Setpro; Westbrook, CT). Interestingly, Szymanski et al. found that the bats that resulted in the greatest increases and decreases in bat velocity were almost identical to that of DeRenne et al. Consequently, because of a smaller subject cohort relative to the research of DeRenne et al. (11), and consistent reports relating to the donut producing the lowest bat velocities, Szymanski et al. concluded that baseball players should avoid using distal loaded bats during warm-ups. The authors also suggested that baseball batters should abide by the guideline of using bats ±12% of a standard baseball bat weight.

Williams et al. (52) also found no statistically significant differences (p > 0.05) between standard, underweighted, and overweighted warm-up conditions on various bat kinematics among 15 National Collegiate Athletic Association (NCAA) Division 1 baseball players. A Vicon Nexus 3D motion capture system (Oxford, United Kingdom) was used to record kinematics. Bat kinematics included maximum resultant velocity (MRV), resultant velocity at ball contact (RVBC), time difference between MRV and RVBC, and bat angles at MRV and RVBC. The authors suggest that bats ranging from 301 g/10.6 oz to 1,576 g/55.6 oz (64.66% weight decrease–85.33% weight increase) do not alter bat swing kinematics. The authors also reported that 73.3% of subjects preferred warming up with the donut attachment.

The study of Wilson et al. (53) exhibited similar findings to that of Williams et al. (52) among 16 NCAA Division II baseball players. Wilson et al. used a SwingProPlus chronograph (Athnetix, Inc., Arcade, New York, NY) to record various measures of bat velocity and acceleration. The authors found no significant effect (p > 0.05) of bat weight (standard, underweighted, and multiple overweighted bats) on PV, PV at PA (PVPA), PA, and time to PA of the standard bat. However, following pooling of the data, a significant time effect was found (p ≤ 0.05) for PV, PVPA, and PA at certain time points post–warm-up. Peak velocity at peak acceleration and PA were not significantly higher (p > 0.05) 1-minute post–warm-up but were significantly greater at 2 minutes (p ≤ 0.05), with both values increasing significantly again at 4 and 8 minutes compared with 2 minutes (p ≤ 0.05). Peak velocity significantly increased at 1 and 2 minutes post–warm-up (p ≤ 0.05), with further significant increases at 4 and 8 minutes compared with 1-minute post–warm-up (p ≤ 0.05). Peak velocity at 8 minutes was also significantly greater than PV at 2 minutes (p ≤ 0.05). The authors concluded that batting performance peaked between 4 and 8 minutes post–warm-up with various weighted bats. This implies that batters should complete their warm-up swings immediately on stepping into the on-deck circle and then use the remaining time until at bat to analyze pitching patterns. However, it is recommended that the reader remains aware of existing criticisms within the literature relating to data pooling. Jenkins (17) states that the pooling of data does not attribute appropriate attention to the existence of variation within and among individuals and likely increases the risk of a type 1 error; that is, rejecting the null hypothesis when the null hypothesis is true (i.e., stating that there is a difference when there is no difference).

As the previous section detailed studies that used a standard 850 g (30 oz) bat, this section details the studies (n = 7) that investigated the acute effects of underweighted and overweighted bats on batting performance with standard bats of different weight (i.e., 822 g/29 oz–964 g/34 oz, excluding 850 g/30 oz) (Table 3). These studies included warm-up implements ranging from 113.40 g (4 oz, 87.5% weight decrease from the study's 907 g/32 oz standard bat) to 2,722 g (96 oz, 317.39% weight increase from the study's 652 g/23 oz standard bat). All 7 studies investigated the effects of such implements on bat velocity, with additional variables measured including resultant swing patterns and subjects' perceptions of bat weight and swing speed on return to the standard bat.

Table 3 - Acute effects of weighted bats on batting performance with alternative standard bats.*
Reference Sport Subjects Implements (% weight increase/decrease from standard) #WU trials; rest period between WU and post-WU; post-WU intertrial rest Results and conclusions
Miller et al. (26) Baseball 32 male recreational players  S 822 g (29 oz)
L 181 g (6.4 oz, 77.93% ↓)
H 1,616 g (57 oz, 96.55% ↑)
3; 2–3 min; 30 s S and L produced significantly faster post-WU BV vs H
Light significantly ↑ BV pre-post and heavy significantly ↓ BV pre-post
Kim and Hinrichs (21) Baseball 20 male experienced players S 885 g (31.2 oz)
H 1,452 g (51.2 oz, 64.1% ↑) donut condition
Arm weight condition
5; 2 minutes; 20 s No significant difference in the effects of WU bats on post-WU BV, but a transient BV ↑ from first post-WU swing to fourth post-WU swing following S and H warm-ups
Rest period of 3-min recommended
Montoya et al. (29) Baseball 19 male recreational players S 893 g (31.5 oz)
L 272 g (9.6 oz, 69.52% ↓)
H 1,565 g donut condition (55.2 oz, 75.24% ↑)
5; 30 s; N/A S and L produced significantly faster post-WU BV (vs H)
Kim and Hinrichs (20) Baseball 13 subjects S 909 g (32.1 oz)
L 113 g (4 oz, 87.5%↓)
H 1,477 g donut condition (52.1 oz, 62.5% ↑)
5; 2 min; N/A No significant differences in BV after using any of the WU bats
Post-WU swings felt “significantly faster” after H WU
Southard and Groomer (41) Baseball 10 experienced players S 964 g (34 oz)
H 1,588 g donut condition (56 oz, 64.71% ↑)
L 340 g (12 oz, 64.71% ↓)
5; 2 min; 15 s Donut condition significantly ↓ BV vs S and L WU's
Weighted bats (bats of ↑MOI) change swing patterns
Szymanski et al. (44) Softball 19 Division 1 intercollegiate players S 652 g (23 oz)
8 devices from 510 g to 2,722 g (18 oz, 21.74% ↓–96 oz, 317.39%↑)
3; 20 s; 20 s No significant differences in BV after using any of the WU bats
Donut produced lowest post–WU BV
Otsuji et al. (33) Baseball and softball 8 university softball and baseball players S 920 g (32.5 oz)
H 1,720 g bat ring condition (60.7 oz, 86.68% ↑
5; 15 s; 15 s No significant BV difference pre–weighted and post–weighted WU.
BV significantly ↓ during first swing post–weighted WU, but perceived S to be lighter and faster during this first post–weighted swing
*L = light bat; H = heavy bat; S = standard bat; BV = bat velocity; WU = warm-up; ↓ = decrease; ↑ = increase; N/A = not available; MOI = moment of inertia.

Similar to DeRenne et al., Southard and Groomer (41) also found the addition of a donut to a standard bat degraded bat velocity (p < 0.001) of 10 experienced baseball players on return to a standard bat. Bat velocity and joint kinematics were measured with a Watsmart Motion Analysis System (Northern Digital, Inc., Waterloo, ON, Canada). A reduced lag between the lead elbow and wrist was revealed, corresponding to a lack of velocity transfer from elbow to wrist. The authors suggest that this alteration resulted in less effective organization of the kinetic chain leading to a greater push motion that potentially contributed to the observed decrease in batting performance. This formed the suggestion that bats with greater moments of inertia alter swing patterns and degrade bat velocity, bolstering the original recommendations of DeRenne et al. (5,8,11). Similar findings were also reported by Montoya et al. (29), whereby an underweighted bat and a standard bat produced significantly faster bat velocities compared with an overweighted bat (p < 0.05). Bat velocity was measured with a custom bat measurement device consisting of 2 vertical photoelectric sensors (Model E3Z; Omron Electronics, Schaumburg, IL).

Similarly, Miller et al. (26) found that bat velocity was significantly slower (p < 0.001) following an overweight condition compared with that of a standard bat condition and an underweighted bat condition. Bat velocity was recorded with a Qualisys 3D motion analysis system (Oqus 210c; Qualisys Motion Technology, Göteborg, SE). No significant differences (p > 0.05) were revealed between the effects of the standard and light bats on post-test bat velocity. Bat velocity of swings post–heavy bat conditions was significantly slower than pre–warm-up swings (p < 0.05), with bat velocity following the light bat significantly increasing from pre–warm-up to post–warm-up swings (p < 0.001).

Kim and Hinrichs (21) found no main effect (p > 0.05) for any of their warm-up conditions (standard bat, overweighted bat, and arm weight condition) on bat velocity of a standard bat, which was measured with an Advanced Motion Measurement–3D system (AMM-3D, Phoenix, AZ). However, the mean bat velocity of the 20 baseball players (high school or college playing experience) participating in the study decreased 0.203 ± 3.83% following the overweighted condition compared with pretest measures, a similar finding to that of Southard and Groomer (41). On further analysis, it was found that bat velocity of the fourth trial following the overweighted bat condition was significantly higher than the first post–warm-up trial (p < 0.05), suggesting that a rest period of 3 minutes is necessary to observe an increase in bat velocity above pre–warm-up velocity measures. The standard bat warm-up also induced a similar immediate decrease in bat velocity (i.e., trial 1–trial 3) followed by a subsequent increase above pre–warm-up values at trial 4. Bat velocity increases at this time were greater than those exhibited at the same time point following the weighted bat condition.

Szymanski et al. (44) reported no significant differences (p > 0.05) in bat velocity following warm-ups with 8 differently weighted bats among a softball cohort, a similar finding to the same research team's aforementioned study with baseball players (45). However, as per the conclusion of their previous study, the authors concluded that softball players should avoid adding the donut to their bat during warm-up swings as the donut condition produced the slowest bat velocity in this study. As in their previous study (45), bat velocity was measured with a Setpro SPRT5A chronograph (Setpro, Westbrook, CT).

In addition to investigating the acute effects of overweighted bats on batting performance, 2 included studies also analyzed subjects' perceptions of bat speed and bat weight post–weighted warm-up. Kim and Hinrichs (20) assessed the effects of various weighted bats (standard, underweighted, and overweighted bats) on standard bat velocity, as well as batters' perceptions of swing weight and swing speed on return to the standard bat. Bat velocity was measured with a Vicon Nexus motion capture system (Oxford, United Kingdom). Although the 13 subjects (8 males and 5 females) experienced no significant differences in bat velocity from pre-test to post-test following the overweighted bat warm-up, subjects' perceived bat speed during the post–warm-ups swings to be faster following the overweighted condition compared with the other conditions.

Similar findings were also reported by Otsuji et al. (33). By measuring bat velocity with 2 photoelectric switches (Model: E3S-3L and EE3S-3D; Omron Ltd., Kyoto, Japan) and a digital data recorder (Model: DR-M3a MK2; TEAC Ltd., Tokyo, Japan), the authors found no significant difference (p > 0.05) in post–warm-up swing velocity following a standard bat condition and an overweighted donut condition. However, bat velocity of the first post-trial following the weighted warm-up was significantly slower (3.3%) than that of the standard condition (p < 0.05) but returned to a similar level as the standard condition in subsequent post-trials. Although such an absence of bat velocity increases was exhibited, subjects perceived post–weighted bat swing speed to be faster and bat weight to be lighter compared with pre-test trials. The findings of these 2 studies indicate the presence of a kinesthetic after effect (KA) following a weighted bat warm-up. A kinesthetic after effect has been defined as the perception of modification to a tool's kinetic properties or perceptual distortion of limb position, movement, or intensity of muscular contraction following the use of the previous implement (31). A kinesthetic after effect has traditionally been considered advantageous to batting performance and potentially gave rise to the popularity of overweighted implements, such as the donut, with the belief that its existence may lead to enhanced performance (11).

Table 4 details the studies that investigated the acute effects of underweighted and overweighted bats on interceptive striking performance with a moving target (n = 4). The dynamic target of these studies included a pitched baseball or the simulation of an oncoming target, with weighted implements ranging from 794 g/28 oz (6.66 and 26.32% weight decrease from the respective studies' standard implement) to 1,531 g/54 oz (80% weight increase from the respective study's standard implement). All 4 studies investigated the effects of respective weighted implements on bat velocity, with additional dependent variables including temporal accuracy, spatial accuracy, swing onset time, subjects' perceptions of bat speed and bat weight on return to the standard bat, and upper limb muscular activity.

Table 4 - Acute effects of weighted bats on batting performance against a moving target.*
Reference Sport Subjects Implements (% weight increase/decrease from standard) #WU trials; rest period between WU and post-WU; post-WU intertrial rest Results and conclusions
Reyes and Dolny (36) Baseball 19 collegiate players S 850 g (30 oz)
L 794 g (28 oz, 6.66% ↓)
H 1,531 g (54 oz, 80% ↑)
18 (3 sets × 6 swings); 30 s; 30 s No WU protocol significantly ↑ BV vs. all other WU protocols but order of S-L-H resulted in greatest BV↑
Nakamoto et al. (31) Baseball 8 male college players S 850 g (30 oz)
H 1,200 g (42.3 oz, 41%↑)
3 or 6; 5 s; minimal H produced marginally (p < 0.10) significant ↑ in post-BV but ↓ temporal accuracy; KA of bat weight but not bat speed post–weighted warm-up
Performers cannot adequately exert perceptual-motor control solely on the basis of practice swings when interacting with moving objects
Weighted tools are not recommended in actual athletic situations
Ohta et al. (32) Baseball 7 male college players S 850 g (30 oz)
H 1,200 g (42.3 oz, 41%↑)
3; 5 s; minimal H produced greater bat speed during V-changed condition
H induced greater timing errors in V-changed condition (vs S), likely caused by a decline in the ability to sufficiently adjust or inhibit muscle activity to correspond with altered target V
H ↓ the adjustment ability associated with inhibition of muscle activation under movement correction conditions
Scott and Gray (39) Baseball E1: 30 experienced players
E2: 20 experienced players
E1: S 1,077 g (38 oz); L 794 g (28 oz, 26.32% ↓); H 1,361 g (48 oz, 26.32% ↑)
E2: S 794 g (28 oz); H 1,077 g (38 oz, 35.71% ↑)
30 (2 blocks of 15 swings); 5 min; immediate E1: Recalibration occurs between 5 and 10 “live” swings following a switch in weighted bats; temporal accuracy degraded
E2: Switching to H significantly ↑ timing errors and significantly ↓ BV vs control condition. Recalibration dependent on the subjects' own individual constraints
Overall: a sudden change in bat weight negatively affects batting; recalibration of perceptual-motor control cannot occur solely on the basis of dry swings
*BV = bat velocity; WU = warm-up; S = standard bat; L = light bat; H = heavy bat; V = velocity; KA = kinesthetic aftereffect; ↑ = weight increase compared with study's standard bat; ↓ = weight decrease compared with study's standard bat; E1 = Experiment 1; E2 = Experiment 2.

In a simulated baseball batting task involving a moving target, Scott and Gray (39) investigated perceptual-motor recalibration when switching between bats of varying weight. The authors reported that although dynamic wielding of a new tool following a change in implement results in immediate recalibration when interacting with a stationary target, the spatiotemporal demands of interacting with a moving target may further diminish the efficacy of using weighted bats immediately before batting performance. Thus, as per the methodologies of the studies in the previous 2 subsections, Scott and Gray (39) imply that the omission of a moving target in previous experimental designs does not sufficiently indicate how a sudden change in bats of differing weight affects batting performance and the perceptual-motor system. Although many of the 18 weighted bat studies included in this section were conducted at a time whereby technology may have not facilitated the presentation of a moving target to subjects akin to those that will be discussed in the current section, 14 of these studies required subjects to perform dry swings or strike a stationary baseball during experimental trials (i.e., absence of a moving target). Therefore, in their dual-experiment study, Scott and Gray (39) had subjects face a computerized batting environment, which simulated a pitcher throwing an oncoming baseball toward the subject. A Fastrak positional tracker sensor was placed on the end of the bat (Polhemus, VT) to determine performance characteristics of each swing. Scott and Gray (39) concluded that switching between bats of different weight results in an inability to recalibrate perceptual-motor control solely on the basis of practice swings (i.e., the absence of bat-ball contact and subsequent performance outcome) due to the persistence of temporal errors. Although subjects recalibrated within 5 (standard to underweighted), 10 (standard to overweighted), or 15 (underweighted to standard) “live” trials (interaction with moving target) post–bat change, the inability of the batter to perform live swings before approaching the plate would result in timing errors of the initial pitches based on this study's results. The authors state that this is due to the fact that until the action begins, the required adjustments on return to the standard bat are variable and unknown, with successful performance dependent on the athlete's ability to recalibrate the relationship between control (force and velocity variables) and perceptual (timing and directional variables) variables. The findings accentuated the importance of performance outcome feedback in the recalibration process when switching between bats of different weight and the dependence of the recalibration process on the subjects' own individual constraints or intrinsic dynamics (i.e., recommended bat weight).

Similarly, Nakamoto et al. (31) investigated the influences of subjective-objective mismatches in bat swings induced by the KA on immediate batting performance against a simulated approaching object. A horizontal trackway with 200 light emitting diodes simulated the continuous motion of the approach target, i.e., a baseball (AO-5N model; Applied Office Co. Ltd., Tokyo, Japan). Kinematic data of the bat were collected with a twentieth-camera digital 3-dimensional motion analysis system (Motion Analysis Corporation, Santa Rosa, CA). Although a KA of increased bat speed has traditionally been considered advantageous, the authors propose that due to consistent reports of decreased bat velocity following overweighted bat warm-ups, subsequent performance with the standard bat may be degraded as a result of actual and perceptual mismatches of bat velocity. Such effects may result in delayed swing onset due to the perception of enhanced bat velocity, mismatching actual decreases of this performance variable. Following a warm-up comprising of 3 swings with a standard bat, 3 swings with an overweighted bat, or a recalibration warm-up comprising of 3 swings with the weighted bat before 3 swings with the standard bat, subjects' batting performance with the standard bat was analyzed when interacting with a simulated oncoming target. Velocity of the bat was collected with a 3D motion analysis system, with interceptive timing performance being determined from the crossover point between the bat and the edge of the trackway. Citing the relatively short-term effect of the KA, Nakamoto et al. used a shorter rest period between weighted bat swings and standard bat swings (<5 seconds) than that of Scott and Gray (39) (5 minutes). Findings indicated a KA of bat weight, but not of bat speed (i.e., actual and perceptual measures of bat speed post–weighted bat increased), with the recalibration warm-up resulting in no change to said perceptions. Although the recalibration warm-up induced smaller absolute temporal errors (ATE) compared with the weighted (p < 0.05) and normal (p < 0.01) conditions in an unchanged-velocity condition; the use of a weighted bat induced significantly greater timing errors (p < 0.05) in a target velocity-changed condition compared with a standard bat warm-up, similar to that of Scott and Gray (39). As no spatial errors were evident, results indicated that the use of a weighted bat had a selective effect on perceptual-motor control requiring movement timing correction.

In reference to the claims in previous studies that weighted bat warm-ups may increase intrinsic muscle contractile properties (e.g., 36), Nakamoto et al. (31) state that the observed selective effect diminishes the efficacy of these prior suggestions as such performance improvements resulting from increased contractile capabilities would be evident regardless of stimulus condition (i.e., spatial or temporal changes of the oncoming target). The authors, therefore, postulated that weighted bat warm-ups affect the central nervous system (CNS), but not the peripheral system, in interceptive striking actions with a moving target. In reference to computational theory, and the reliance on predictive mechanisms to correct movement due to the rapid nature of interceptive striking tasks, Nakamoto et al. (31) suggest that weighted bat warm-ups result in the inability of batters to correctly alter swing performance during a velocity-changed task due to inadequate error detection contributing to the distortion of the efference copy. As evident in their results, this produces faster swings and larger timing errors, as well as inducing motor programming errors resulting in such movements that could not be adjusted online. Thus, the authors conclude that weighted bat warm-ups negatively affect the performance of motor skills that greatly rely on anticipation and that practice swings alone are not sufficient to recalibrate and exert effective perceptual-motor control during a dynamic interceptive striking action.

With the same experimental design as their previous study (31), the same research team (32) completed a follow-up study that investigated the effects of a weighted bat warm-up on bat velocity, subjects' perceptions of bat weight and speed, temporal and spatial accuracy, as well as upper limb muscle activity during post–warm-up swings with a standard bat. Muscle activity was measured with an EMG device (SynaAct MT-11; NEC Sanei Inc., Japan), whereby bipolar surface electrodes were placed parallel to muscle fibers on the muscle belly of the right and left pectoralis major, biceps brachii, triceps brachii, flexor carpi radialis, and the extensor carpi ulnaris. Subjects performed 3 maximal effort swings with either a standard bat or an overweighted bat. This study supported the previous findings (31) as the weighted bat resulted in the perception of increased bat velocity and decreased bat weight, with neither warm-up implement degrading spatial accuracy (i.e., location-changed condition). Swing velocity for both warm-up conditions was significantly lower in the velocity-changed condition compared with the unchanged condition (p = 0.019, d = 3.18). However, in line with subjects' perceptions of bat speed, bat velocity in each of the 3 conditions was faster following the weighted bat warm-up, replicating the finding of the previous study (31). Although there were no significant differences (p > 0.05) in ATE in the velocity-changed condition following either the weighted or normal warm-up, the greater ES (d = 1.61) pertaining to ATE following the weighted condition illuminated a negative effect of the use of the overweighted bat during warm-up swings (i.e., decreased temporal accuracy). The authors used an ES index calculated in accordance with Cohen's d index. Following the weighted warm-up, significantly greater ATE was evident in the velocity-changed condition compared with the unchanged condition (p = 0.024, d = 3.48) and the location-changed condition (p = 0.001, d = 2.79). There was no significant effect (p > 0.05) of the weighted bat warm-up on muscle activity before ball impact during the velocity or spatial changed conditions compared with the unchanged condition.

In the velocity-changed condition post–weighted warm-up, subjects coordinated decreased bat velocity to accommodate the change in target velocity by inhibiting extensor carpi ulnaris (ECU) activity. This muscle functions in ulnar flexion of the wrist, thus aiding bat velocity. However, a lower degree of difference in ECU activity between the velocity-changed and unchanged condition was evident following the weighted warm-up (5.9% ± 7.40) compared with the standard warm-up (12% ± 4.16). These findings suggest that overweighted bats result in an acute decline in the adjustment ability of muscle activity as ECU activity is insufficiently inhibited to correspond to a change in target velocity. Although this supports their previous findings that suggested weighted bat warm-ups affect the CNS (31), the authors state that they were unable to dismiss the potential of inducing a potentiation effect due to the use of an overweighted bat warm-up. However, corresponding to the absence of significant increases of muscle activity post–weighted bat warm-up, it was postulated that the spatiotemporal demands and accompanying uncertainty of oncoming target trajectory and velocity likely resulted in submaximal effort swings, thus decreasing the likely expression of potentiated neuromuscular performance. Consequently, the authors conclude that weighted bat warm-ups affect coincident timing performance in tasks involving a velocity change due to CNS interference as indicated by a decline in the ability to sufficiently adjust or inhibit muscle activity. Such implements do not result in peripheral system interference as indicated by an absence of significant differences in EMG measures, a potential result of subsequent submaximal effort swings to correspond with the spatiotemporal demands of an oncoming target.

With the inclusion of a moving target in post–weighted bat trials, Reyes and Dolny (36) assessed the acute effects of various warm-up procedures comprising a standard bat, an underweighted bat, and an overweighted bat. Interestingly, this study found a warm-up protocol following the order of standard-light-heavy produced the greatest bat velocity increase (6.03%), followed by a protocol comprising solely of heavy bat swings (+5.08%). Bat velocity was measured with 2 infrared photocell control boxes (Model #63504, Lafayette Instrument, Lafayette, IN) and a multifunction timer (Model #54035A; Lafayette Instrument, Lafayette, IN). However, as per the acknowledgment of Scott and Gray (39), performance accuracy was not measured with no reports of experimental trial swing errors included. The findings of Nakamoto et al. (31), Ohta et al. (32), and Scott and Gray (39) collectively imply that although overweighted bats potentially increase bat velocity, temporal accuracy reductions leading to degraded performance are likely to occur.

This review process identified that studies investigating the acute effects of weighted implements on interceptive striking are conducted almost exclusively in the baseball domain. Of the 18 included studies in this section, 17 investigated the effects of weighted bats on baseball batting performance. This emphasizes an absence of research into the acute effects of other weighted implements that may aid interceptive striking actions, such as weighted golf clubs, weighted hockey sticks, and weighted racquets. In relation to studies included in this section, this review provides further support to the recommendations of DeRenne et al. (11) to use bats within ±12% of a standard 30 oz bat. In particular, distal loaded tools (for example, the donut) and bats with increased moments of inertia consistently resulted in decreased bat velocity, altered swing patterns, and inaccurate perceptions of bat velocity and weight on return to the standard bat. There are a limited number of studies investigating the acute effects of overweighted bats that have exhibited the potential to increase bat velocity; in contrast, such tools negatively impact interceptive striking of a dynamic target due to the creation and persistence of temporal accuracy errors that are not modulated by dynamic wielding of the standard tool. Because of the range of included weighted implements, and variation in weight of regulation bats permitted among baseball populations, results of these studies warrant careful consideration. As per the cohorts of the respective studies, some subjects may have been using a standard bat that was overweighted compared with their respective bat in competition. Therefore, it is recommended that, if feasible, practitioners determine the effects of respective weighted bats on each individual's batting performance. However, because of the inability of batters to practice live performance before taking the field, the ineffectiveness of dynamic wielding as shown in this review sheds light on the potential negative effects of overweighted bats. These studies (31,32,39) highlight the implications of previous studies that did not include a moving target and the subsequent effects of these implements on markers of performance beyond bat velocity, muscular activation, and batters' perceptions of bat properties and speed. However, future research is warranted that further investigates the acute effects of underweighted bats on interceptive striking of a dynamic target.

Multiple studies included in this review have made suggestions as to the underlying mechanisms responsible for the justification of using weighted implements as a warm-up strategy and the observed effects on return to the standard implement. Based on their findings, Van Huss et al. (50) suggested the use of overweighted implements increases motor unit activation, with Morimoto et al. (30) suggesting the use of underweighted implements results in enhanced neuromuscular activity. Wilson et al. (53) suggested that Banister Fitness Fatigue Model may sufficiently explain their findings as the greatest increases in batting performance were observed between 4 and 8 minutes. This model proposes that performance is a balance between fitness and fatigue, whereby changes to the former outlast those of the latter following recent contractile activity (23). This implies that maximal performance does not occur immediately following the training stimulus, thus explaining the delayed enhancement of performance following a weighted implement protocol as exhibited in the study of Wilson et al. (53). This model may also explain the findings of other included studies who experienced similar delayed performance enhancements (21,50).

Wilson et al. (53) also referred to the possible elicitation of postactivation potentiation (PAP) following the use of overweighted bats as a potential mechanism for subsequent performance. Indeed, Judge et al. (18) and Bellar et al. (2) attributed observed performance increases to this phenomenon following their investigations into the acute effects of overweighted indoor weight throw implements. Postactivation potentiation is defined as the transient increase in short-duration contractile force capabilities of a high-velocity, short-duration competitive movement as a result of previous contractile activity of relatively higher intensity (27). This phenomenon has been suggested as the most appropriate physiological evidence of the acute effects of the fitness-fatigue model (23). Specifically, the realization of PAP is a function of the net balance between potentiation and fatigue, subsequent to the imposed conditioning activity (37). In reference to previous studies (29,45), Wilson et al. identified the studies' short rest periods as a likely factor in the absence of improved performance following the weighted implement warm-up protocols. Had these aforementioned studies included greater rest times such as those of Wilson et al. (53) (4–8 minutes) and Kim and Hinrichs (21) (≥3 minutes), improved performance may have been observed. Wilson et al. (53) state that the ≤1-minute rest periods of these studies would have resulted in any existing potentiation being dominated by fatigue within the respective time frame as per the fitness-fatigue model. In the performance environment, however, predicting when the respective individual's at-bat will take place to begin weighted bat warm-up swings may not be practical due to the unpredictable nature of the preceding batter's at-bat situation. As highlighted by Wilson et al., coaches often advise batters to initially take some pitches during their at-bat to identify the types of pitches the pitcher is throwing. This, therefore, adds to the variability in time the preceding batter may spend at-bat, and varies the time the subsequent batter may spend in the on-deck circle. As a result of this, exploring different time periods in which to begin weighted bat swings in the on-deck circle, within the time frame of the time periods that facilitate improved performance as identified by Wilson et al. (53) and Kim and Hinrichs (21) (i.e., ≥3 minutes or 4–8 minutes), is suggested (53).

As verification of PAP requires observation of increased peak twitch force/torque and increased rate of twitch force/torque development, which are verified via electrical stimulation of single muscles/muscle groups (3,24,35), it may, however, be suggested that Banister model (23) and postactivation performance enhancement (PAPE) (35) are the most appropriate explanations of the acute effects of weighted implements based on the recommended mechanisms of the included studies. However, although these models may sufficiently explain reports of enhanced batting performance following a weighted bat warm-up in studies using a stationary target, the findings of studies assessing such effects when intercepting a dynamic target potentially diminish the efficacy of attaining such potentiated peripheral states. Ohta et al. (32) state that because of the spatiotemporal demands in live batting environments (i.e., with a moving target), it is likely that batters correspondingly perform submaximal effort swings, thus alleviating the likelihood of expressing such potentiated efforts.

The conclusions of the included studies collectively highlight varying suggestions as to the underlying mechanisms of weighted implement training and accompanying effects on immediate performance. Therefore, future research is necessary to identify these underlying mechanisms and the efficacy of their elicitation in representative task designs, particularly in interceptive striking sports. An investigation into the interaction of such mechanisms and various rest periods should also be conducted to aid weighted implement warm-up design and further the field's understanding of their acute effects on sports performance.

This systematic review highlights an overriding, and almost exclusive, prevalence for investigating the acute effects of weighted baseball implements on the sport's respective motor skills (i.e., batting and pitching). This highlights a need for future research to investigate the acute effects of weighted balls on respective motor skills in other sports such as soccer, rugby, basketball, golf, and American football. It also revealed a need for future research to assess the acute effects of weighted implements on interceptive striking performance in other sports, such as golf, field hockey, ice hockey, and all racket-based sports. Such insights will inform and expand weighted implement training literature, as well as potentially facilitate the design of preparatory strategies to enhance immediate performance with the respective standard competitive implement.

Although the use of weighted implements in preparation for competition has traditionally been considered advantageous, this review highlights both positive and negative implications of their use immediately before competitive performance. From the limited number of studies investigating the acute effects of weighted balls on pitching and bowling performance, findings indicate that, in certain conditions (consideration of weight of modified implement, volume of repetitions performed, and time interval between weighted and standard implement repetitions), there is potential for enhanced pitching velocity. However, consideration for the effect of weighted implements on pitching accuracy is required. The 4 studies that assessed the acute effects of weighted implements on pitching and bowling accuracy reported an absence of significant change or a detrimental effect on throwing accuracy. The 2 studies investigating the acute effects of weighted indoor weight throw implements on indoor weight throw performance imply that positive effects on subsequent standard implement throw distance may be realized. However, there is a need for future research to expand the weighted implement training research among the track and field events and to identify the optimal programming of these implements indicating the optimal weight, repetitions, and rest times needed to realize significant improvements in performance. In contrast, research investigating the acute effects of weighted bats on batting performance is more ambiguous with various studies inferring negative effects of such implements. The use of overweighted bats has consistently been demonstrated to negatively impact bat velocity and overall batting performance, with further suggestions that bats of greater moments of inertia (distal loaded) negatively affect bat velocity and swing patterns (41). Furthermore, their effects on immediate motor skill performance may not be isolated to objective measures of performance but may also influence batters' perceptions of regulation bat swing speed and heaviness, potentially resulting in an objective-subjective mismatch (a kinesthetic after effect) that has been shown to have detrimental effects on batting success. Existing literature has identified the need for live swings to immediately recalibrate to the standard bat so that potential enhancements in batting performance following a weighted bat protocol can be realized (31,39). Consequently, these studies suggest that the use of weighted implements before competitive performance is not recommended. This finding has implications for future experimental designs when investigating interceptive striking performance in sports with spatiotemporal demands due to the prominence of moving targets. However, 7 of the 12 studies (58%) that included an underweighted bat found significant increases in standard bat velocity with bats as light as 6.4 oz (26,29). It is, therefore, recommended that more representative experimental set-ups similar to those of Nakamoto et al. (31), Ohta et al. (32) and Scott and Gray (39), be used to further investigate the acute effects of underweighted bats on batting performance with coincident timing demands.

Practical Applications

The findings of this systematic review suggest that particular weighted implements induce acute performance increases in subsequent performance using the standard competition implement. For example, in athletics, the use of +1.37 and +2.27 kg overweighted implements immediately before competition may facilitate increased throw distance among high school, collegiate, and elite athletes, that is, 12.77 and 13.67 kg for male high school athletes, 17.24 and 18.14 kg for male collegiate and elite athletes, 10.47 and 11.37 kg for female high school athletes, and 10.44 and 11.34 kg for female collegiate and elite athletes. Although both overweighted implements improve throw distance, the +1.37 kg implement may produce optimal results. With regard to weighted bats, the results of this review suggest that the use of underweighted bats, as light as 181 g (6.4 oz) and 272 g (9.6 oz), may increase subsequent standard bat velocity. However, further research is needed to investigate the acute effects of warm-up swings with an underweighted bat on standard bat performance when attempting to intercept a dynamic target (i.e., a pitched baseball). Pitching weighted baseballs before competitive pitching performance may increase pitched-ball velocity, but careful consideration should be given to its potential negative effects on pitching accuracy. Additional consideration, on the part of the strength and conditioning coach or skill acquisition specialist, should also be given to each individual's strength level as it is suggested that this is an important variable to assess when considering the efficacy of using weighted implements as a warm-up strategy (2,18).

Acknowledgments

No external sources of funding were provided during this research. The authors report no conflict of interest.

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Keywords:

weighted implement training; warm-up; overload; underload; skill acquisition

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